Treatment of magnesium and its alloys



Patented June 27,- 1939 UNITED. STATES 2,163,583 TREATMENT OF MAGN'EsIUMAND ITS ALLOYS Robert/W. Buzzard, Kensington, Md.

No Drawing. Application November 26, 1937,

Serial No. 176,651

6 Claims. (01. 148-6) (Granted under the act of March 3, 1883, asamended April 30, 1928; 3'10 0. G. 757) This invention relates to theforming of protective coatings on magnesium or its alloys.

It is an object of this invention to regulate the pH of a bath so as toform on magnesium or its 5 alloys a protective coating that hasexceptionally good mechanical and corrosion-resisting characteristics.

It has been proposed in United States Patent 1,947,122 to treatmagnesium or magnesium alloys with a solution of a dihydrogen phosphateand a salt of chromic acid in order to form an adherent coating on thesurface of the metal to protect the same against corrosion. A solutionprepared in accordance with this patent gives approximately a 1! neutralbath slightly on the acid side, however the pH of this bath is notadjusted and regulated and is at all times higher than applicant's bathherein set forth. This bath produces a light gray coating on Dowmetal Halloy consisting of 6 percent aluminum, 2 percent zinc, and theremainder magnesium. I have found that a distinct improvement in and asuperior corrosion resistant coating on magnesium and its alloys can beobtained by forming the coatings on the metal surfaces in a bathcontaining phosphoric acids and chromic acids or compounds thereof withthe pH adjusted to and maintained within the range of pH 0.5 to pH 3.5.The resultant coating as produced by my process varies from a deep blackto a greenish black on H alloy denoting a difference in physicalproperties from the specific coating referred to in the aforementionedpatent. Further, a corrosion test of H alloy specimens disclosed thatthe specimens prepared in accordance with the patent failed 75 percentof the coating in 200 hours, whereas, the specimens treated inaccordance with my method herein described failed 10 percent of thecoating over the same period.

The bath of my improved process consists essen- 40 tially of solutionsof phosphoric acid and chromic acid adjusted to a pH within the range ofpH 0.5 to pH 3.5 by a compound giving the basic reaction. To adjust thepH I add hydroxide, preferably ammonium hydroxide, however, eithersodium or potassium hydroxide may be used, to the solution until theproper value is obtained.

Practically any concentration of chromic acid and phosphoric acid hasbeen found satisfactory as long as the pH is adjusted, the usefulconcentration apparently being limited only by the solubility of thesalts used, although some concentrations are preferable for particularalloys and for particular types of coatings.

The article to be coated is thoroughly cleaned, a clean surface beingessential, to the formation of good coatings. Degreasing, electrolyticcleaning, acid cleaning, and acid pickling previous to coating all aidin the preparation of the surface of the metal for a good coating,although a number of known pretreatments may be used to clean the 5metal. fter cleaning and washing, the article to be coate is immersed inthe solution heated-to approximately the boiling point of water orabove. The time of treatment depends upon the strength of the solutionand the quality of coating desired. 10 Coatings have been producedbyimmersing the article for periods varying from 1 minute to 2 hours.

As an example, an article of magnesium alloy is first degreased, washed,electrolytically cleaned, 15 washed, cleaned by acid dip, washed andthen immersed for 30 minutes in a bath containing 90 grams/liter chromicacid and grams /liter phosphoric acid adjusted to pH 1.5 with ammoniumhydroxide, the bath being maintained during 20 treatment at atemperature substantially equiva-- lent to or above the boiling point ofwater. The bath, if and when necessary, is regulated and maintained atthe proper pH by adding either phosphoric or chromic acid to the bath.25

The invention herein described and claimed may be used and/ormanufactured by or for the Government of the United States of Americafor governmental purposes without the payment of any royalties thereonor therefor. 30

I claim:

1. A method of forming a protective coating on magnesium and magnesiumalloys, comprising immersing the metal to be coated in a bath containingchromic acid and phosphoric acid adjusted 35 to a pH within the range ofpH 0.5 to pH 3.5.

2. A method of forming a protective coating on magnesium and magnesiumalloys, comprising immersing the metal to be coated in a bath containingsubstantially grams per liter chromic 4c acid, 80 grams per literphosphoric acid and sumcient ammonium hydroxide to adjust the bath to apH 1.5 and maintaining a constant pH of 1.5 by adding an acid selectedfrom the group of phosphoric acid and chromic acid. 45

3. A method of forming a protective coating on magnesium and magnesiumalloys, comprising immersing the metal to be coated in a bath containingchromic acid, phosphoric acid and hydroxide selected from the groupammonium hy- 50 droxide, sodium hydroxide and potassium hydroxide foradjusting the bath to a pH within the range pH 0.5 and pH 3.5, said bathbeing maintained during treatment at a temperature substantiallyequivalent to or above the boiling point 55 of water and beingmaintained at all times within the range pH 0.5 and pH 3.5 by adding anacid selected from the group of phosphoric acid and chromic acid.

4. A method of regulating -a bath for treating magnesium and magnesiumalloys comprising making a bath containing chromic acid and phosphoricacid, adjusting said bath within the range of pH 0.5 and pH 3.5 andmaintaining at all times the bath within the range of pH 0.5 and pH 3.5.

5. A method of forming protective coatings on magnesium and magnesiumalloys and for regulating a bath for treating the metal, comprisingmaking an aqueous solution containing chromic acid and phosphoric acid,adjusting said solution within the range of pH 0.5 and pH 3.5 by addinga water solublesubstance selected from the group of alkaline salt andhydroxide to the solution and maintaining the resultant bath at alltimes within the range of pH 0.5 and pH 3.5 by adding an acid selectedfrom the group of chromic acid and phosphoric acid.

6. A method 0! forming protective coatings on magnesium and magnesiumalloys and for regulating a bath for treating the metal, comprisingmaking an aqueous solution containing chromic acid and phosphoric acid,adjusting said solution within the range of pH 0.5 and pH 3.5 by addinghydroxide selected from the group of ammonium hydroxide, sodiumhydroxide, and potassium hydroxide to the solution and maintaining theresultant bath at all times within the range 01' pH 0.5 and pH 3.5 byadding an acid selected from the group of chromic acid and phosphoricacid.

ROBERT W. BUZZARD.

